The invention relates to temperature indicating devices and more particularly to such devices for the visual indication that the device has been maintained at or above a predetermined temperature over a predetermined period of time.
Many materials, such as for example, photographic film, pharmaceutical and biological products, perishable foods and the like, must be stored under the prescribed temperature conditions and for limited periods of time in order to retain the freshness or effectiveness of the product. Normally, however, the appearance of the product is such that the consumer or user cannot determine whether the product has been properly stored or is still in a useful condition at the time of purchase or selection for use.
To overcome this problem various temperature indicators, or telltales, have been developed to indicate the temperature history of a product. Typically, these devices include a temperature sensitive material which flows at or above a preselected temperature for contact with a suitable indicator material to visually indicate that the device and presumably the product has been exposed to temperatures exceeding the design for storage temperature.
Although generally such devices are designed only to indicate that a particular temperature has been exceeded, some prior art telltale devices have been designed to relate both time and temperature.
One such device is disclosed in U.S. Pat. No. 2,782,749 and is based upon the use of temperature sensitive materials having non-specific melting points, such as fats and waxes, which gradually become less viscous as temperature increases. These temperature sensitive materials are formed as a layer on a porous paper barrier and as the temperature increases, the fat or wax begins to soften and eventually diffuses through the paper and reaches a point of visibility on the opposite side of the barrier. The diffusion rate of the material through the barrier is directly related to the temperature. Devices of this type, due to the nature of the fats and waxes, have a relatively narrow temperature range at which they can be used and it is difficult to control the diffusion rate through the paper barrier. In addition, the paper barrier must be relatively thick, on the order of 0.75 inches, so that the device is relatively bulky and may give rise to handling and storage problems when attached to packages or containers.